1. Field of the Invention
The present invention relates to an optical disk, a tracking error signal generating apparatus, and a tracking control apparatus. More particularly, the present invention is concerned with a high-recording-density optical disk using a sampled servo method, a tracking error signal generating apparatus for generating a tracking error signal from such an optical disk, and a tracking control apparatus for performing tracking control of such an optical disk.
2. Description of the Related Art
There is a recording format based on a sampled servo method as a recording format of an optical disk.
FIG. 1 shows a recording format of an optical disk of the sampled servo method. The optical disk based on the sampled servo method does not have any pre-grooves (guide grooves) on a recording film of the optical disk, but servo areas (fields) at 1376 points in one track are pre-formatted. The optical disk based on the sampled servo method is characterized in that a tracking error and a clock for recording/reproducing can be generated by sampling by means of the pre-format.
As shown in FIG. 1, a signal track having a spiral form extending from the inner portion of an optical disk DK to the outer portion thereof is formed in a program area PA of the optical disk DK. One track is divided into 32 sectors. Each of the sectors includes 43 segments, and each of the segments contains 18 bytes. In the first segment #0 of one sector are pre-formatted a sector synchronizing signal S.sub.sync (two bits) for establishing synchronization for each sector and a sector address S.sub.ADR (16 bits) for indicating the address of the above sector. The above pre-formatting is performed in the process of mastering of the optical disk. Each of the segments #1 to #42 consists of a 18-byte area including a two-byte servo area F.sub.S and a 16-byte data area F.sub.D.
FIG. 2 shows a recording format of the servo area F.sub.S, The two-byte servo area F.sub.S is segmented into two servo bytes #1 and #2. A first wobble pit P.sub.W1 is pre-formatted at bit of the servo byte #1, and a second wobble pit P.sub.W2 is pre-formatted at the eighth bit thereof. As shown in FIG. 2, the position of the first wobble pit P.sub.W1 is located at the third bit in 16 tracks (A), and is indicated as P.sub.W1A. The position of the first wobble pit P.sub.W1 is located at the fourth bit in 16 tracks (B), and is indicated as P.sub.1B. In this manner, since the position of the first wobble pit P.sub.W1 is changed every 16 tracks, the number of tracks crossing during searching can be correctly detected.
The first wobble pit P.sub.W1 and the second wobble pit P.sub.W2 are disposed so that these pits are respectively shifted, by 1/4 of the track pitch, from a track center TC in a direction (the radial direction of the writable optical disk DK) perpendicular to the tracing direction. A tracking error detection is performed on the basis of the difference between the quantity of a return light obtained at the first wobble pit P.sub.W1 and the quantity of a return light obtained at the second wobble pit P.sub.W2. A clock pit CP for synchronization is pre-formatted at the 12th bit in the servo byte #2. A space between the second wobble pit P.sub.W2 and the clock pit CP has a mirror surface and has a clock length equal to 19 channels. Synchronization can be established for each segment by counting this 19 channel clock pre-formatted in the above space. Focus error detection is also carried out during the synchronization detection period. FIG. 2 shows a signal S.sub.T1 (S.sub.T1A or S.sub.T1B) for use in tracking and the sector synchronizing signal S.sub.sync, these signals being obtained by reading the servo area F.sub.S by the laser beam.
A description will now be given, with reference to FIG. 3, of a method for detecting a tracking error by means of wobble pits. A reference A indicates a first case where the read beam runs on the center axis (track center axis) between a pair of wobble pits P.sub.W1 and P.sub.W2. An RF signal obtained in the first case is indicated as S.sub.A. When the read beam runs near the wobble pit P.sub.W1 or P.sub.W2, a small quantity of reflected light is obtained due to the optical diffraction effect, and the reflected light becomes dark. When the read beam passes just on the clock pit CP, the darkest reflected light is obtained. A reference B indicates a second case where the read beam moves on a portion deviating from the track center axis towards the inner circle end of the optical disk. The RF signal obtained in the second case is indicated as S.sub.B. In the second case, since the read beam passes just on the wobble pit P.sub.W1, the dark portion by the wobble pit P.sub.W1 is darker than that by the wobble pit P.sub.W2. A reference C indicates a third case where the read beam moves on a portion deviating from the track center axis towards the outer circle end of the optical disk. The RF signal obtained in the third case is indicated as S.sub.C. The RF signal S.sub.C has a waveform reverse to that of the RF signal S.sub.B.
It is assumed that SAMPLE (T.sub.1) indicates a signal value obtained by performing signal sampling at the time of the wobble pit P.sub.W1, and SAMPLE(T.sub.2) indicates a signal value obtained by performing signal sampling at the time of the wobble pit P.sub.W2. The difference between the SAMPLE(T.sub.1) and the SAMPLE(T.sub.2), that is, SAMPLE(T.sub.1)-SAMPLE(T.sub.2) is equal to zero, a negative value and a positive value in the first, second and third cases A, B and C, respectively. Assuming that SAMPLE(T.sub.1)-SAMPLE(T.sub.2)=TE, the TE can be used as a tracking error signal.
In the above-mentioned conventional sampled servo method, the wobble pits P.sub.W1 and P.sub.W2 as well as the clock pit CP are per-formed on the optical disk (pre-pits), and a variety of information for use in servo control, such as a tracking error signal, is generated by the arrangement of these pits.
In the information reading operation, the laser beam reflected by the signal pit PT is also diffracted by the signal pit PT, and a small quantity of light returns to the optical pickup therefrom, so that the position of the signal pit PT is handled as a dark portion. On the other hand, the space between the signal pits PT has a mirror surface, and the laser beam is totally reflected by the mirror surface. Hence a large quantity of light returns to the optical pickup, and the corresponding portion is handled as a light portion. In order to correctly read servo information, it is necessary to read the above darkness and lightness without any error. In order to read darkness and lightness without error, conventionally, as shown in FIG. 4A, it is necessary to design a track pitch width T.sub.P (approximately 1.6 .mu.m, for example) so that it is greater than the diameter B.sub.L of a spot formed by the laser beam.
In the above case, in order to improve the recording density of the optical disk DK, it may be considered to reduce the track pitch width. FIG. 4B or FIG. 4C shows the track pitch width T.sub.P reduced to a half (approximately 0.8 .mu.m) of the conventional width. The difference becomes small between the quantity of light obtained in an on-track state shown in FIG. 4B, in which the center of the laser beam is located on the track center axis, and the quantity of light obtained in an off-track state shown in FIG. 4C, in which the center of the laser beam is located out of the track center axis, and hence the servo control cannot be correctly performed. As a result, it is impossible to reduce the track pitch width beyond a limited value. It may be considered to shorten the wavelength of the laser beam and diminish the size of the pits in order to improve the recording density of the optical disk DK. However, also in this case, there is a limit regarding the track pitch due to the spot diameter B.sub.L. Further, it becomes difficult to record the wobble pits at high speed and high precision.